CMS-PAS-HIN-18-016

CMS-PAS-HIN-18-016
Nuclear modification factor of isolated photons in PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV
CMS Collaboration
November 2019

Abstract: The transverse energy ( $E^{\gamma}_\text{T}$ ) spectra and the nuclear modification factors ( $R_{{\mathrm{AA}}}$ ) of isolated photons are analyzed using proton-proton (pp) and lead-lead (PbPb) collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV. The data were collected using the CMS detector at the CERN LHC with corresponding integrated luminosities of 404 $\mu$ b $^{-1}$ and 27.4 pb $^{-1}$ for PbPb and pp data, respectively. The results are shown as a function of photon $E^{\gamma}_\text{T}$ in the pseudorapidity range $\|{\eta}\| <$ 1.44, for $E^{\gamma}_\text{T}$ between 25 and 200 GeV and for different centrality bins. The data are compared to JEPHOX next-to-leading-order calculations and found to be in good agreement. In addition, $R_{{\mathrm{AA}}}$ is consistent with unity for all centralities in PbPb.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 07 (2020) 116. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Template fit of the shower shape variable $\sigma _{\eta \eta}$ for 40 $< {E^\gamma _\text {T}} <$ 50 GeV in the 10-30% centrality class. The black points show the PbPb experimental results. The red histogram is the signal template obtained from PYTHIA+HYDJET simulations, and the green histogram is the background template estimated from the data for the non-isolated sideband region.
png pdf	Figure 2: Efficiency of the isolated photon detection as a function of ${E^\gamma _\text {T}}$ for PbPb in 0-10% centrality (left) and for pp (right). The different colors represent various selection criteria.
png pdf	Figure 3: Isolated photon spectra (top) measured as a function of $E^\gamma _\text {T}$ for 0-10%, 10-30%, 30-50%, 50-100% and 0-100% PbPb collisions (scaled by $T_\text {AA}$ ) at 5.02 TeV. The spectra are scaled by the factors shown in the legend for a clearer display. The horizontal bars indicate the bin width. The total systematic uncertainties are shown as boxes in each $E^\gamma _\text {T}$ bin. The spectra in the 0-100% centrality bin are compared to the NLO JETPHOX calculations with EPPS16 nPDF. The ratio of the data to JETPHOX is shown in the lower panel. The yellow boxes indicate the total systematic uncertainties of the data. The blue and red hatched boxes correspond to the JETPHOX PDF and scale uncertainties, respectively. The markers are placed at the center of the bin.
png pdf	Figure 4: Isolated photon cross section (top) measured as a function of $E^\gamma _\text {T}$ in pp at 5.02 TeV. The horizontal bars indicate the bin width. The total systematic uncertainties are shown as boxes in each $E^\gamma _\text {T}$ bin. The data are compared to the NLO JETPHOX calculations with CT14 PDF. The ratio of the data to JETPHOX is shown in the lower panel. The yellow boxes indicate the total systematic uncertainty of the data. The blue and red hatched boxes correspond to JETPHOX PDF and scale uncertainties, respectively. The markers are placed at the center of the bin.
png pdf	Figure 5: Nuclear modification factor $R_\text {AA}$ as a function of the photon ${E^\gamma _\text {T}}$ measured in the 0-10%, 10-30%, 30-50% and 50-100% centrality ranges in PbPb. The vertical bars associated with symbols indicate the statistical uncertainties and the horizontal bars reflect the bin width. The total systematic uncertainties without the $T_\text {AA}$ uncertainty are shown as the colored boxes. The $T_\text {AA}$ uncertainty, common to all points for a given centrality range, is indicated by the gray box centered at unity on the left side of each panel. The luminosity uncertainty of pp collisions is shown as the brown box at unity at the left-most position. The markers are placed at the center of the bin.
png pdf	Figure 6: Nuclear modification factor $R_\text {AA}$ as a function of the photon ${E^\gamma _\text {T}}$ measured in the 0-100% centrality range in PbPb. The vertical bars indicate the statistical uncertainties and the horizontal bars reflect the bin width. The total systematic uncertainties without the $T_\text {AA}$ uncertainty are shown by the colored boxes. The $T_\text {AA}$ uncertainty, common to all points, is indicated by the gray box centered at unity on the left side of each panel. The luminosity uncertainty of the pp data is shown as the brown box at unity at the left-most position. The NLO JETPHOX calculation with the ratio of EPPS16 nPDF with CT14 PDF baseline to CT14 is compared to the data. The blue and red hatched boxes correspond to JETPHOX PDF and scale uncertainties, respectively. The markers are placed at the center of the bin.

Tables
png pdf	Table 1: Average numbers of the nuclear overlap function ( ${T_{\rm AA}}$ ) and their uncertainties for various centrality ranges used in this analysis.
png pdf	Table 2: Summary of the contributions from various sources to the estimated systematic uncertainties on the cross section of isolated photons in pp and PbPb. When ranges are shown, they indicate the $E^\gamma _\text {T}$ -dependent variations of the uncertainties.
png pdf	Table 3: Summary of the contributions from various sources to the estimated systematic uncertainties on the nuclear modification factors in pp and PbPb. When ranges are shown, they indicate the $E^\gamma _\text {T}$ -dependent variations of the uncertainties.

Summary

The differential cross section and nuclear modification factor of isolated photons are reported in pp and PbPb collisions at

${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV with the CMS detector, at midrapidity (

$|{\eta^{\gamma}}| <$ 1.44) for photon transverse energy from 25 GeV to 200 GeV. The data are compared with the next-to-leading order perturbative QCD calculations using JETPHOX for CT14 PDF and EPPS16 nuclear PDF sets. The prediction from JETPHOX is found to be consistent with the measured

$R_{{\mathrm{AA}}}$ and cross sections for both pp and PbPb collisions.

No significant modification of isolated photon cross sections in PbPb collisions with respect to pp collisions is observed in the explored kinematic ranges at various collision centralities. The current measurements significantly improve the accuracy compared to the previous CMS results. We conclude that the isolated photons are not affected by the strongly interacting medium produced in heavy ion collisions, and they can be valuable probes to access the initial parton's

${p_{{\mathrm{T}}}}$ in

$\gamma$ +jet or

$\gamma$ +hadron events.

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